1. Technical Field
The present invention relates to a solid oxide fuel cell device, and more particularly to a solid oxide fuel cell device furnished with a load following function for changing the amount of fuel supplied in accordance with the amount of required power load.
2. Description of the Related Art
The most important issue in attaining a practical fuel cell device is how to achieve the two-fold goal of preventing fuel cell breakage and saving energy (reduce electrical grid power from commercial power sources and increase generated power from fuel cells).
Research is currently underway toward the development of practical solid oxide fuel cell (also referred to below as “SOFC”) device. The SOFC device operates at relatively high temperatures, using an oxide ion-conducting solid electrolyte as an electrolyte, with electrodes placed on each side thereof, supplying fuel gas on one side and oxidizer (air, oxygen, or the like) on the other.
In such SOFC device, because the volume of hydrogen and air supplied to the fuel cells are extremely minute prior to reaching the state in which the hydrogen (fuel) and oxygen supplied to the fuel cells are being stably supplied to the entirety of the fuel cells (e.g., to 160 fuel cells connected in series), the problem arises that time is required until uniformity in the supply of hydrogen and air amounts is achieved in each fuel cell. An additional problem was the long time required until the target electrical generating reaction could be stably conducted in all of the fuel cells, due to factors such as individual differences and temperature differences between the fuel cells. In addition to the problems of reformer hydrogen reform delay and non-achievement of the hydrogen reform volume target values, the problem also arose in the SOFC device that time was required for the process of reaching the ideal state, due to these various difficult-to-control and uncertain elements.
From one perspective, because SOFC electricity cannot be sold to utilities it is necessary from an energy saving standpoint to perform load-following control, whereby the amount of fuel supplied is made to follow changes in power required of the fuel cell device, which in turn is determined by user (general households, etc.) demand power, and varies with time of day and the like. However, when load following is implemented there is a risk that because of changes in items such as the supply amounts of fuel, air, and water, the amounts of fuel and air supplied to individual fuel cells will be nonuniform, or the flow volumes supplied to the reformer will be different from target values, etc. There is also a risk that large differences in the amount of electricity generated will arise between individual fuel cells because of temperature changes in the fuel cells associated with load following control. The above-described unstable conditions can lead to severe situations in which fuel cells fail.
To resolve such problems, JP-07-307163-A discloses a fuel cell device (a phosphoric acid fuel cell device) in which power is output by instructing an inverter permitted current value to the fuel cell, using a delay time after instructing a gas increase or decrease amount determined by the amount of change in load; in the method of JP-07-307163-A, breakage of fuel cells caused by fuel depletion can be suppressed, since during load following power is not extracted until the amount of fuel is ideal. However, because this type of time delay occurs when extracting electrical power, load following characteristics are degraded, so from an energy saving standpoint, this solution alone is not enough. The fuel cell of JP-07-307163-A is thus unable to solve the dual problem of increasing energy saving performance and preventing breakage of fuel cells.
In addition, JP-3353406-B discloses a following-type fuel cell device in which AC output is controlled by obeying externally supplied load commands. The fuel cell device of PJP-3353406-B uses feedback control to perfectly follow output current from the fuel cell relative to changes in load; in addition it also restricts the rate of change in output current from exceeding a predetermined rate.